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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 15 |
| 4 | 63 |
| 5 | 237 |
| 6 | 1034 |
| 7 | 3945 |
| 8 | 17577 |
| 9 | 67640 |
| 10 | 304902 |
| 11 | 1179415 |
| 12 | 5352038 |
| 13 | 20771331 |
| 14 | 94628132 |
| 15 | 368083879 |
| 16 | 1680820301 |
| 17 | 6548692260 |
| 18 | 29946087674 |
| 19 | 116816782997 |
| 20 | 534628747310 |
[1,4,15,63,237,1034,3945,17577,67640,304902, 1179415,5352038,20771331,94628132,368083879, 1680820301,6548692260,29946087674,116816782997, 534628747310] |